The present invention relates generally to the evaluation of circuit element characteristics, e.g. capacitance, resistance and inductance, of integrated circuits (ICs) and various other kinds of electronic devices, and relates more particularly to improper signal transmission between components in devices for evaluating such characteristics.
FIG. 2 shows an example of prior art apparatus for measuring circuit element characteristics that is specifically arranged to obtain IC characteristics. Control device 101, which may be a computer, has input/output terminals 106, 107 and 108 which are connected through optical fiber cables 115, 116 and 117 respectively to a terminal 109 of a signal generating device 102, to a terminal 110 of signal measuring device 103 and, to a terminal 111 of pulse generating device 104. Signal generating device 102, signal measuring device 103 and pulse generating device 104 are connected to a circuit 105 to be tested in order to measure the values of applied voltage and current flowing within circuit 105. The connections from devices 102, 103 and 104 to circuit 105 are made via coaxial cables 118, 119 and 120 which connect terminals 112, 113 and 114 to the measurement terminals 121, 122 and 123 of circuit 105. Control device 101 is connected to ground via a power source (not shown in FIG. 2). The chassis of signal generating device 102, signal measuring device 103 and pulse generating device 104 are connected to each other, and the assembly is connected to a common ground through a power source (not shown in FIG. 2), so that the reference potential of signal generating device 102, measuring device 103, and pulse generating device 104 is nominally the same. The circuit 105, the device under test, is also connected to ground.
The apparatus shown in FIG. 2 operates as follows. To measure the A.C. characteristics, e.g. capacitance, inductance and resistance, of circuit 105, first of all, control device 101 sends a control signal to signal generating device 102 through optical fiber cable 115. In response to the control signal, signal generating device 102 provides an A.C. signal with a predetermined frequency to circuit 105 through cable 118. Control device 101 also sends a control signal to signal measuring device 103 through optical fiber cable 116, directing device 103 to measure the voltage produced in circuit 105. The measurement data is transmitted to control device 101 for further calculation to obtain the A.C. characteristics of circuit 105, also via optical fiber cable 116.
To measure digital characteristics, e.g. slew rate or threshold level, pulse generating device 104 is used to produce the test signal applied to circuit 105, and a coordinated measurement of the response is made by device 103 in a manner similar to the one used for A.C. measurements, above. The pulse generating device 104 is connected to circuit 105 via wire 120.
Measurement error can be introduced by fluctuations of the reference potential transferred between control device 101, signal generating device 102, signal measuring device 103 and pulse generating device 104. When several devices are connected to a common reference potential, the fluctuation of a reference potential in one device can cause voltage noise which may transfer to the other devices as a voltage drop caused by current flowing through wires connecting the devices. These fluctuations make it impossible to measure circuit characteristics of a device under test with high accuracy.
In order to prevent such measurement error, optical fiber cables can be used to electrically isolate two devices, while transmitting signals between them. Thus, in FIG. 2, optical fiber cables 115-117 prevent transfer of ground potential voltage fluctuations from control device 101 to signal generating device 102, signal measuring device 103 and pulse generating device 104. The photocouplers which send and receive the optical signals through the optical cable have response characteristics suitable for digital signals which are not adversely affected by any distortion of the waveform or noise. They are not suitable, however, for synchronizing signals and measuremnt signals, especially wide band high frequency AC signals, which are adversely affected by noise and waveform distortion and thus cannot be transmitted over optical fiber cables. Therefore, optical fiber cables cannot be used to replace the coaxial cables 118-120 used in prior art for transmitting the high frequency test signal or the measurement signals which are typically in the VHF band.
Since signal generating device 102, signal measuring device 103, pulse generating device 104 and circuit 105 remain connected to a common reference potential and connected to device 105 through the outer conductors of coaxial cables 118, 119 and 120, the measurement accuracy may be degraded by reference potential fluctuations in any of these devices.
One method for lessening the effect of reference potential noise is to use shielded twisted pair cables conveying signals in balanced mode in place of the coaxial cables 118, 119 and 120. At one end of the shielded pair cable, the shielding outer conductor is not connected and the twisted pair is terminated by an isolation pulse transformer. This method is of limited application, however, because its narrow bandwidth for signal transmission. The isolation transformer limits bandwidth to 2-3 decades, too narrow for measuring IC characteristics.
Another method uses balanced signal transmission emitter-coupled logic (ECL) circuits to terminate the cables. The disadvantage of this method is its low common mode impedence because of the low terminating resistance of ECL circuits.